Subscribers not receiving voice mail

We are using Unity 5.0. A voice mailbox was created. Most people can leave voice mail in the box without a problem. Some people can only leave voice mail in the box about 50% of the time. One person we've identified can never leave voice mail in the box. When an unsuccessful attempt is made, caller hears the correct prompt, and message of about 10 seconds is left. The ports all jump just fine, although we don't see MWI port dial out to the phone. A report from Exchange shows other messages left for the voice mailbox but not the ones that don't show up. The voice mail does not show up in the dead letter folder.